A spectrometer is a device which receives a light signal as an input and produces as an output a light signal which is spread out in space according to the different wavelength components, or colors, of the input light signal. A detector attached to the spectrometer analyzes the output signal, called the spectrum, to quantify the amount of each wavelength component which is present in the input signal. One specific type of spectrometer is known as an Offner spectrometer which can be used to produce images of a remote object over a contiguous range of narrow spectral bands. This type of imaging is known as hyperspectral imaging and has recently emerged as an important part of the military/aerospace solution to airborne and spaceborne reconnaissance and remote sensing. Basically, the hyperspectral imaging system utilizes an Offner spectrometer and an advanced data processing technology to produce imagery with embedded spectral signature data. This signature data is useful in a wide-variety of applications such as target designation/recognition, missile plume identification and mine detection (for example). In addition, the hyperspectral imaging system can be used in a wide-variety of commercial applications such as cancer detection, environmental monitoring, agricultural monitoring and mineral exploration.
A conventional hyperspectral imaging system includes a fore optic for receiving an image from a remote object. The fore optic focuses the image onto a fixed slit. The slit transmits a slice of the image (line of light, trimmed image) to a spectrometer (e.g. prism, diffraction grating), which disperses the light according to wavelength and directs it to a two-dimensional image sensor (e.g. FPA detector) for recording.
One of the limitations of conventional hyperspectral imaging systems is the time required to acquire a two-dimensional image of a scene over a wide area. The limitation is a consequence of the basic design of conventional hyperspectral imaging systems. Conventional systems include a single fore optic and a single fixed slit for receiving and transmitting images to the spectrometer for dispersion and detection. A single fixed slit limits imaging to a single line of light from the remote object, which means that only a limited area of the remote object can be imaged at a time. In addition, a hyperspectral imaging system with a fixed slit is limited to filling only those pixels on the image sensor that correspond to the narrow line image defined by the fixed slit. Commercially-available image sensors typically have a much greater number of pixels than is needed to detect the dispersed light corresponding to the slice of the remote object that is transmitted as the line image originating from the fixed slit. Poor pixel utilization makes image collection inefficient.
To improve the spatial field coverage at a particular resolution, prior art systems may aggregate multiple conventional hyperspectral imaging systems. Multiple hyperspectral imaging systems can be positioned side-by-side such that the linear fields of view of each system are contiguous and imaging of contiguous slices of the remote object can occur in parallel to improve collection efficiency. This solution, however, is impractical for many applications due to the space, weight, and power constraints, as well as the costs of the multiple detectors, coolers, spectrometers, and other components needed for implementation.
There are currently two techniques for extending the hyperspectral imaging capacity of a single hyperspectral device from a single line of light (single slice) of the remote object to a two-dimensional area (multiple slices) of the remote object. The first technique involves moving the entire hyperspectral imaging system in a direction perpendicular to the fixed slit and synchronizing the image taking with that motion to obtain the hyperspectral image of a wide area of the remote object. This technique is often called the “push broom” method. The second technique involves placing a rotating mirror in front of the imaging lens of the fore optic and synchronizing image collection with the motion of the mirror to obtain the hyperspectral image of an area of the remote object.
One of the primary issues encountered in generating a hyperspectral image using the above approaches is the time required to generate a complete image. In either the push broom or rotating mirror methods, a single line of spatial image data passes through the slit and is spectrally interrogated. To sample additional line images (slices) of the scene, the field of view visible in the slit must be systematically moved to adjacent lines of the image (either by adjustment of the scanning mirror, or translation of the imager relative to the object being interrogated), and the process must be repeated until the complete scene is generated line by line. Given the desired high spatial resolution of the images, this method requires hundreds if not thousands of individual line scans to be taken one at a time and combined into a master two-dimensional image.
Although traditional hyperspectral imaging system and traditional techniques for obtaining the hyperspectral image of an area of the remote object may work well in some applications, it is desirable to develop new hyperspectral imaging systems that can be used to obtain the hyperspectral images, especially 2D or areal images, of the remote object. It is particularly desirable to develop hyperspectral imaging systems that are compact, lightweight, and capable of quickly providing an areal image of a scene over a wide area.